10.1 Diseases and immunity

2026 Syllabus Objectives

  1. Describe a pathogen as a disease-causing organism
  2. Describe a transmissible disease as a disease in which the pathogen can be passed from one host to another
  3. State that a pathogen is transmitted:
    • (a) by direct contact, including through blood and other body fluids
    • (b) indirectly, including from contaminated surfaces, food, animals and air
  4. Describe the body defences, limited to: skin, hairs in the nose, mucus, stomach acid and white blood cells
  5. Explain the importance of the following in controlling the spread of disease:
    • (a) a clean water supply
    • (b) hygienic food preparation
    • (c) good personal hygiene
    • (d) waste disposal
    • (e) sewage treatment (details of the stages of sewage treatment are not required)
  6. Describe active immunity as defence against a pathogen by antibody production in the body
  7. State that each pathogen has its own antigens, which have specific shapes
  8. Describe antibodies as proteins that bind to antigens leading to direct destruction of pathogens or marking of pathogens for destruction by phagocytes
  9. State that specific antibodies have complementary shapes which fit specific antigens
  10. Explain that active immunity is gained after an infection by a pathogen or by vaccination
  11. Outline the process of vaccination:
    • (a) weakened pathogens or their antigens are put into the body
    • (b) the antigens stimulate an immune response by lymphocytes which produce antibodies
    • (c) memory cells are produced that give long-term immunity
  12. Explain the role of vaccination in controlling the spread of diseases
  13. Explain that passive immunity is a short-term defence against a pathogen by antibodies acquired from another individual, including across the placenta and in breast milk
  14. Explain the importance of breast-feeding for the development of passive immunity in infants
  15. State that memory cells are not produced in passive immunity
  16. Describe cholera as a disease caused by a bacterium which is transmitted in contaminated water
  17. Explain that the cholera bacterium produces a toxin that causes secretion of chloride ions into the small intestine, causing osmotic movement of water into the gut, causing diarrhoea, dehydration and loss of ions from the blood

Pathogens and Transmissible Diseases

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🔑 Key Definitions

Pathogen: A microorganism (a tiny organism that can only be seen with a microscope) that causes disease.

Host: An organism in which a pathogen lives and reproduces.

Transmissible disease: A disease that can be passed from one host to another; transmissible diseases are caused by pathogens.

Types of Pathogens

Many diseases are caused by pathogens that get into our bodies and breed there. There are four main types of microorganisms that can act as pathogens:

Group of PathogenExamples of Diseases
VirusesInfluenza, common cold, poliomyelitis, measles, AIDS
BacteriaCholera, syphilis, whooping cough, tuberculosis, tetanus
ProtoctistsMalaria, amoebic dysentery
FungiAthlete's foot, ringworm

How Pathogens Cause Disease

Once inside the body, pathogens cause harm in two main ways:

  1. Direct cell damage: Some pathogens damage our cells by living in them and using up their resources.

  2. Toxin production: Others produce waste products called toxins, which spread around the body and cause symptoms such as:

    • High temperature
    • Rashes
    • General feeling of illness

Important Note: Some toxins produced by pathogens are among the most dangerous poisons in the world – for example, the one produced by the bacterium Clostridium botulinum.

HIV Virus Structure

HIV (human immunodeficiency virus) is approximately 0.1 μm0.1 \, \mu\text{m} across and consists of:

  • Red areas: Genetic material
  • Blue circles: Protein coats

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